Bowlers, like any other athlete, are always looking for a competitive advantage. One area in which this advantage is constantly sought is in the equipment used by the bowler. And of the equipment used by any specific bowler, the most important is the bowling ball.
For the better part of a century, bowling balls have been manufactured of two or more parts. An inner part is termed the "core" while the outer part is termed the "cover". Not infrequently, the core itself is made up of more than one part.
Core density and/or core geometry exert significant effects over the reactive characteristics of any given ball. The term "reactive characteristics" is commonly employed to denote the degree to which a ball will hook, i.e. deviate from the line of travel on which it was originally released, and how soon the hooking action begins to occur as the ball is rolling down a lane. A highly reactive ball will hook more than a less reactive ball, all other things being equal. A cover heavy ball will begin to hook further down the lane and/or change direction more slowly than a center heavy ball. That is to say, center heavy balls are said to react earlier in their path of travel whereas cover heavy balls are said to react later in their path of travel.
A variety of factors influence the selection of a ball by a bowler. A bowler who releases the ball with a relatively low velocity frequently will prefer a cover heavy ball so that the reaction does not occur to soon such that the ball hits too high in the pocket or even misses it entirely. Conversely a bowler who rolls a relatively high velocity ball will prefer a center heavy so as to assure that the desired reaction will occur sufficiently early that the ball will not slide by the pocket.
Of course, a large variety of factors other than core weight and geometry have a considerable affect on the reactivity of a ball. The material of which the cover is made quite obviously has an effect because of differing effect on the frictional characteristics of the surface of the ball. This invention, however, is not concerned with the effects of such variables. Rather, it is concerned with providing a core of novel geometry and with providing a family of bowling balls, at least some of which are of significantly varying weight, that have substantially identical reaction characteristics.
Typically, when a new ball having some sort of desired reactive characteristics is developed, great effort is applied to the details of coverstock formulation and the shape of the weight block of the sixteen pound prototype ball. Sixteen pound balls are chosen for the prototypes because they facilitate testing by professional and other high quality bowlers who almost exclusively use a sixteen pound ball.
Once the sixteen pound design is finalized as a result of such development and testing, lighter weight balls are produced by removing weight from the core of the sixteen pound design. Some manufacturers even change the core shape that was developed for the sixteen pound ball when making lighter weight balls in favor of any convenient shape with no regard for rotational dynamics. Weight is removed from the core as opposed to the cover stock because the resin and filler systems customarily employed to make cores can produce a much wider range of densities than the resin systems used to produce cover stock. However, removing weight from a core even while maintaining the same core shape produces bowling balls with different rotational dynamics and differing reaction characteristics.
Still another technique used to make lighter weight balls involves the use of a compound core design. Typically, this involves the use of a high density inner core and a low density outer core. With this technique, the ratio of cover density to core density that exists in a sixteen pound ball can be maintained in the ratio of the outer core density to the inner core density in the lightweight balls. Stated another way, the lightweight ball made according to this technique employs lower densities in the core to lower the weight of the ball but the cover to core density ratio is maintained. This represents a significant improvement over the method of simply removing weight from the core but even so, it fails to preserve the same rotational dynamics at ail weights. This is due to the fact that the cover stock remains of the same density for ail ball weights which in turn causes the reaction characteristics of the lighter weight balls to differ from those of the sixteen pound version.
According to the invention, the aforementioned problems with lightweight ball designs are avoided by designing lightweight balls so that they have proportionally the same relationship between their moment of inertia and the ball/lane frictional forces as a sixteen pound ball of the same family.
As is well known, the moment of inertia, which is the resistance to a change in the rotational state of an object, plays an important part in bowling ball reaction as explained more fully in U.S. Pat. No. 5,074,553. Further, dynamic or sliding fiction between the bowling ball and the surface of the bowling lane is substantially responsible for the forces which cause a ball to hook or curve as it travels down the lane. Dynamic fiction is, of course, directly proportional to the weight of the ball in the case of a bowling ball rolling on a bowling lane so that as the weight of the ball is decreased, the resulting frictional force is also decreased proportional to the decrease in weight.
By way of example, consider a twelve pound ball. The same is twenty-five percent lighter than a sixteen pound ball. To achieve substantially identical reaction characteristics (assuming identical cover stock, identical surface finish, identical position of grip holes relative to the core of the ball, the maximum, minimum and differential moments of inertia of the twelve pound ball would have to be approximately twenty-five percent lower than those of the sixteen pound ball.
Typically, lightweight balls have a lower maximum moment of inertia than does a heavier ball. However, the decrease is usually not proportional to the decrease in weight and the result is that lightweight balls have different rotational dynamics than heavier balls and as a consequence, differing reaction characteristics.
The practice of removing weight from the core while maintaining the same core shape is to be particularly avoided. A fifteen pound ball cast with the same internal core shape typically will lose more than twenty-five percent of the differential moment of inertia of the sixteen pound version. For a 14 pound ball, the loss will be sixty-six percent. This decrease of differential moment of inertia results in the lighter balls having different rotational dynamics than heaver weight balls and therefore, different reaction characteristics.
According to the present invention, proportional lowering of the moments of inertia and differential moments of inertia can be accomplished by maintaining the same radii of gyration and differential radius of gyration for all the balls in a family of balls of greatly varying weight.